TECHNICAL FIELD

The subject invention relates to a reaction vessel used for implementation of mechanochemical reactions, and which is suitable for in-situ recording of Raman spectra of samples in reaction vessels during the milling or grinding procedure on mills. The invention also relates to the method of recording Raman spectra using the reaction vessel of the invention.

Technical Problem

Mechanochemical reactions are commonly implemented by means of mechanochemical mills which can be vibration ball mills, planetary ball mills etc. Reaction vessels for mechanochemical reactions for the implementation of reaction comprise the corresponding reactants and additives and one or more grinding balls and are completely closed at the moment of grinding. The mill is a device which propels the reaction vessels by causing the relative movement of reaction vessel and steel balls, which as a consequence has the mixing of reactants and hitting of balls against the reactants. The advantages of mechanochemical reactions in relation to reactions performed in solutions are reduced or minimal use of solvents, quick and energy non-demanding reactions and quantitative product yield. Mechanochemical reactions may be performed in a way that reaction vessels with reactants and grinding balls are installed on the mill, which then oscillates or rotates the reaction vessels. With vibration ball mills the reaction vessels oscillate with the typical frequency of 30 Hz. In one performance of vibration mill, the mill crank holding the reaction vessel oscillates from its left end point to its right end point in the horizontal plane. In another performance of vibration ball mill, oscillation of reaction vessel happens in the vertical plane, i.e. reaction vessel moves up and down.

Until recently, the reaction vessels were made exclusively of nontransparent materials such as steel, agate, tungsten carbide, zirconium oxide, aluminium oxide and similar, which disabled in-situ monitoring of physical and chemical changes that happened during the implementation of a mechanochemical procedure. For example, nontransparent reaction vessels disable the penetration of incident laser beam, necessary for obtaining Raman spectra of sample, within reaction vessel where the laser beam should interact with the reaction mixture. When using nontransparent reaction vessels, the incident laser beam cannot pass through the wall of the reaction vessel.

Monitoring and understanding of the course and processes occurring by implementation of mechanochemical reactions is desirable from the aspect of optimization of mechanochemical reactions. Until the development of the first in-situ techniques for monitoring the mechanochemical reactions without halting the grinding process, the monitoring of the course of mechanochemical reactions resided on halting of grinding process for the purposes of opening the reaction vessel and sampling of the reaction mixture. Such an approach to the study of the course of mechanochemical reaction is inconvenient because some mechanochemical reactions are not halted by cessation of the grinding process but continue in the reaction mixture obtained in such a way. Since the subsequent analysis which follows after the sampling and preparation of sample for adequate analysis inevitably runs late after the moment of sampling, so every sample change after the sampling gives wrong information about the course of the studied mechanochemical reaction and can lead to erroneous conclusions about the advancement and progress of mechanochemical reaction. Besides, the grinding process heats the reaction mixture, which is the consequence of the transformation of kinetic energy of the grinding balls into the inner energy of the reaction mixture and also of the reaction vessel itself. Halting of the grinding process and its separation into segments for the purposes of sampling indisputably causes cooling of the reaction mixture and the reaction vessel and its re-heating by the continuation of the grinding procedure. Because of this, temperature changes of the reaction mixture are different when it comes to interrupted grinding process and when it comes to uninterrupted grinding. It is clear that for the application of mechanochemical reactions, it is desirable to perform the process uninterruptedly since the same one needs less time, but also less human work. It is already clear from these reasons that the approach to following the mechanochemical reactions of grinding, which demand the halting of grinding process, is defective and it cannot provide relevant information about the processes which happen during grinding. Therefore, the development of techniques for monitoring of mechanochemical reactions which do not demand halting of the grinding process is desirable for a better understanding of those processes and later for their optimization for the purposes of obtaining the desirable products within the shortest period of time and with the least consumption of other resources such as energy and raw materials. Two techniques that exist today and that enable monitoring of mechanochemical reactions without interrupting the grinding process are based on either the diffraction of high energy X-rays or on Raman spectroscopy.

Raman spectroscopy is a recognized method of sample characterization and is often applied in scientific and applied research including chemical, pharmaceutical, biochemical, biological, and physical research. It is a spectroscopy method where the laser beam interacts with the sample and non-elastic scattered radiation is analyzed. Raman scattered radiation is only a small fraction of radiation which is scattered on the sample. Approximately one out of a million photons from the incident laser beam scatters non- elastically on the sample giving the Raman signal. Therefore, it is of crucial importance to enable the collection of as many as possible Raman scattered photons when scattering happens during grinding and the incident laser beam passes through the wall of reaction vessel in order to interact with the sample and the scattered radiation leaves the reaction vessel in order to be collected and analyzed using a spectrometer. By collecting Raman spectrum of a sample, in principle, we are gaining insight into the vibrational spectrum of the sample depending on the sample composition. Different samples have, therefore, different Raman spectra and the changes in the sample composition can be measured and monitored over time, by recording the time-resolved Raman spectra. In a standard manner, Raman spectra are recorded in a way that incident laser beam falls directly on the sample and Raman scattered radiation is collected directly from sample. For the application of Raman spectroscopy on mechanochemical reactions, direct contact of the incident laser beam with the sample is not possible since the reaction mixture is contained in the reaction vessel which oscillates during the grinding process.

More recently, for the implementation of the mentioned mechanochemical reactions in mills transparent reaction vessels are used which enabled in-situ monitoring of chemical reactions by using the diffraction of X-ray radiation. Such reaction vessels are typically made of poly(methyl metacrylate) (glass-plastics). Such reaction vessels consist of two complementary parts that make the closed reaction vessel when joined and from the outside typically have the form of a cylinder whereas from the inside they close the chamber which is completely rounded. Such transparent reaction vessels can be used for recording Raman spectra of the reaction mixture in-situ during the implementation of a mechanochemical procedure.

For recording Raman spectra the incident laser beam passes through the wall of the transparent reaction vessel and enters the reaction vessel where it interacts with the sample or the reaction mixture. The incident laser beam scatters on the sample and one portion of scattered radiation pertains to Raman scattered radiation which has altered energy from the energy of the incident monochromatic radiation. Raman radiation is of interest for this patent application since it is characteristic for chemical composition of the sample and by changing the chemical species different Raman spectrum is obtained. By changes in the Raman spectrum of the reaction mixture, monitoring of the reaction during the grinding procedure is enabled. For example, as long as the spectrum of the reaction mixture changes, reaction is still ongoing under the influence of grinding. The termination of alterations in spectrum of the reaction mixture indicates the termination of the chemical reaction and completion of the reaction. By this in-situ Raman spectroscopy can be used for determination of the time necessary for the completion of some mechanochemical reaction and by this mechanochemical procedure can be optimized.

However, although the sole use of transparent reaction vessels enables the obtaining of spectrum from sample during grinding and without halting the grinding procedure, it was shown that such reaction vessels are not suitable for monitoring the course of mechanochemical reactions at reaction mixtures which provide weak effect of Raman scattering. Therefore, there was a need for improvement, i.e. for solution which would enable higher quality in-situ recording of Raman spectra by mechanochemical reactions.

State of the art

The use of a plastic reaction vessel for implementation of mechanochemical reactions was described earlier [scientific paper: Friscic et al. Nature Chem., 5 (2013) 66-73] . The material of reaction vessel which was used in that scientific paper is polymethylmetacrylate (PMMA, also known under the names glass-plastics, Plexiglas or Perspex). Raman's spectra are collected in-situ and without halting the grinding procedure and were also collected by using such transparent reaction vessels [scientific papers: Gracin et al. Angew. Chem. Int. Ed. 53 (2014) 6193-6197; Batzdorf et al. Angew. Chem. Int. Ed. 54 (2014) 1799-1802, Juribasic et al. Chem. Commun. 50 (2014) 10287-10290; Tireli et al. Chem. Commun. 51 (2015) 8058-8061] .

Raman's spectra were recorded by using non-transparent reaction vessels but which have a transparent window.

In this way, European patent application EP2784488A1 relates to reaction vessel for Raman spectrophotometry and a method using such vessel. On page 3, line 4, paragraph (0016) the reaction vessel from drawing 1 to 5 is described. It is mentioned that housing portion 10 includes transparent window portion 11 , through which the inner portion of reaction vessel 12 in which electrolyte solution E is obtained is visible. That window serves for Raman spectrophotometry of electrochemical reaction happening in the reaction vessel. Such reaction vessel is not adapted to work in a grinding procedure.

US patent application US2004/0166310 relates to the use of stabilized zirconium oxide for window through which different chemical reactions would be followed in different reaction vessels. According to patent claim 10, such window is used, among others, also for optical methods of monitoring reactions including spectroscopy. Patent application does not indicate the possibility of usage of such reaction vessel for grinding procedures.

Description of the invention

The subject invention represents the enhanced reaction vessel for in-situ recording of Raman spectra of mechanochemical reaction mixtures.

The invention relates to reaction vessels for in-situ Raman spectroscopy of samples during the grinding procedure on mills where the outer surface of the reaction vessel is of cylindrical shape and the inner surface of the reaction vessel in completely rounded and where the cylindrical portion of outer surface of reaction vessel has one or more flat transparent faces on the mantel of the vessel cylinder for passing of the incident laser beam to the inside of the reaction vessel and leaving of the Raman scattered radiation. It is desirable that the mills are vibration mills.

The reaction vessels of the subject invention enable the collection of improved Raman spectra during the grinding procedure compared to currently known reaction vessel for the collection of Raman spectra in grinding procedures. The enhancement of Raman spectra relates to approximately 2.3 times larger ratio of the Raman signal from the sample and the Raman signal from the reaction vessel when reaction vessels of the subject invention are used compared to currently known transparent reaction vessels (Drawings 4a, 4b and 4c and Table 1 ). In addition, Raman signal of the sample is approximately 25 % larger when reaction vessel of the subject invention is used where the incident laser beam falls into the reaction vessel vertically through its flat portion in relation to the reaction vessel with cylindrical outer mantel. Also, Raman spectroscopy can be performed in-situ, without the need to halt the grinding process or to open the reaction vessel containing the sample or to take the sample out of the reaction vessel in order to record its Raman spectrum.

By using reaction vessels of the subject invention, a better ratio of the Raman signal of the reaction mixture and noise and also better ratio of the Raman signal of the reaction mixture and the Raman signal of the reaction vessel was achieved. Namely, the material itself of the transparent reaction vessel gives Raman signal which is impossible to avoid since the beam on its way to the reaction mixture in the inner portion of the reaction vessel partially interacts with the material of the vessel and partially disperses on the material of the reaction vessel. However, with an altered design of the reaction vessel, Raman signal of the sample in relation to noise and in relation to the Raman signal of the reaction vessel can be enhanced and that is the subject of this invention.

It is desirable that the reaction vessel is wholly made of transparent material. Transparent material can be selected, among others, from poly(methyl metacrylate), polyethylene or polycarbonate.

It is desirable that the area of the flat face on the outer surface of the reaction vessel is such that it is sufficient to allow for the incident laser beam to enter the reaction vessel perpendicularly through that flat and transparent face/wall of the reaction vessel. Namely, the area of the flat face of the reaction vessel of the subject invention should be large enough so that the laser beam falls onto it for the whole time while reaction vessel of the invention is moving on the mill. According to the usual design of laboratory mills and the amplitude of movements of the crank holding the reaction vessel on the mill, flat transparent face is long from 1 mm to the whole length of the cylinder of the reaction vessel. It is desirable that the length of the flat face of cylinder is from 10 mm to 20 mm. It is important that the flat transparent face of reaction vessel is at least of the length that the reaction vessel travels from the right end point to the left end point when it is installed on the mill.

The width of the flat face should be large enough so that the laser beam falls into the reaction vessel through this flat face in all positions in which the reaction vessel can be found relative to the laser beam during the grinding procedure. Accordingly, in all positions of the reaction vessel from the left end point of the mill's crank to the right end point. The width of the reaction face on the reaction vessel can be within the range from 1 mm to 20 mm. It is desirable that the width of the flat face ranges from 3 mm to 10 mm.

It is desirable that on the reaction vessel there is only one flat transparent part of surface sufficient enough so that the laser beam can pass through. In that case, the rest of cylindrical part of the reaction vessel could be easily placed on the mill's crank and that the mechanochemical reactions that occur by shaking of the mill could further be performed efficiently.

The width of the wall of reaction vessel should be as small as possible in order that Raman signal from reaction vessel in Raman spectra would be as small as possible, but still sufficient enough so that the reaction vessel has adequate firmness. It is desirable that the width of the wall of reaction vessel be adapted to the properties of the incident laser beam. For example, in performances when the incident laser beam is focused at certain distance from the top of laser probe, it is desirable that the width of the wall of the reaction vessel, at the position where the reaction vessel has its flat face, is less than that certain distance of the laser beam focus from the top of the laser probe. By doing so it is achieved that the laser beam focus is inside the reaction vessel where the reaction mixture is situated and Raman spectra of which are measured.

In case that the transparent reaction vessel of the invention consists of two complementary parts which are joined in a way that they create the closed reaction vessel of curved and smooth inner shape, it is desirable that both complementary parts of the reaction vessel have flat surface on the outer surface of the reaction vessel which are of the same widths so that by closing the reaction vessel one continuous flat face at the outer surface of the closed reaction vessel is obtained.

Performance is possible in which the flat portion of the closed reaction vessel does not extend through the whole length of reaction vessel but the reaction vessel is wholly curved on its ends which touch the mill's crank when the reaction vessel is installed on the mill. In that case each of the two complementary parts has a flat portion of the outer wall extending from the openings of each half of the reaction vessel. In that way, when the reaction vessel is closed, the total length of the flat surface of the outer wall of the closed reaction vessel is equal or larger than the amplitude that the reaction vessel has while installed on an operating mill. In that way it is achieved that the laser beam falls through the flat portion of reaction vessel in all positions of the reaction vessel during milling.

It is desirable that the reaction vessel with flat face during the experiment of collection of in-situ Raman spectra is used in a way so that the incident laser beam enters the reaction vessel perpendicularly through flat face. Although it is possible to collect Raman spectrum by using the reaction vessel with completely curved outer wall, if the laser beam enters vertically into the reaction vessel through the flat and transparent wall, the high-quality Raman spectrum of the sample is obtained. The comparison of Raman spectra obtained by different ways of recording is shown in drawings 4a, 4b and 4c. The results of the analysis of the signal strength which were obtained in these three ways of recording are given in Table 1.

The subject invention also relates to the method of collection of Raman spectra of the reaction mixture during the grinding procedure and without halting the grinding process, which uses the reaction vessel of the subject invention in a way that reaction vessel of the subject invention is installed on the vibration ball mill so that the flat transparent face is in horizontal flat surface and turned downwards. Thereby, the laser probe is installed below the reaction vessel in a way that the incident laser beam enters the reaction vessel vertically through its flat surface and Raman scattered radiation is collected in a way that it exits the reaction vessel through the same flat face. Brief description of drawings

Drawing 1 - Technical drawing of the reaction vessel on which flat portion is visible on the surface of the reaction vessel

Drawings 2a and 2b - Technical drawings of the half of the reaction vessel, i.e. one complementary part of the reaction vessel on which flat portion on the surface of reaction vessel is visible

Drawings 3a and 3b - Represent lateral and sideways section of the reaction vessel

Drawings 4a, 4b and 4c - Represent the results of examination of reaction vessels with curved outer surface and flat outer surface

DETAILED DESCRIPTION OF AT LEAST ONE WAY OF EMBODIMENT

One of desirable embodiments of the reaction vessel of the subject invention is described below.

In this embodiment of the reaction vessel of the subject invention reaction vessel 1 consists of two complementary portions 7 which are connected so that they form closed reaction vessel of curved and smooth inner shape. Each of complementary portions 7 has one part of the outer wall flat whereas the remaining portion of the reaction vessel is curved 3. Two complementary portions 7 are connected in a way that in the assembled and closed reaction vessel, flat portions of both parts are aligned so that the flat portion of the closed reaction vessel 2 is obtained on the whole length of the reaction vessel 1.

The closed reaction vessel 1 is 65 mm long and is suitable for mounting on standard mechanochemical mills. The radius of the reaction vessel on the part where the reaction vessel is curved is 12.5 mm from the axis of the reaction vessel to the outer edge. The straight portion of the wall of the closed reaction vessel 2 is of the width„b"=9 mm and is situated throughout the whole length of reaction vessel„/". On the portion where the flat surface of reaction vessel 2 is situated, the width of the wall of reaction vessel„d" is smaller than in the portion where the wall of the reaction vessel is curved. The distance from the central axis of the reaction vessel to the outer flat wall of the reaction vessel 2 is 11 .5 mm.

The inner surface of the reaction vessel 4 is of cylindrical shape in the central part of the reaction vessel whereas on the ends they are hemispheres. In the inner cylindrical part of the reaction vessel, diameter of reaction vessel is 9.5 mm. By doing so, it is achieved that the inner wall of reaction vessel 4 is without sharp edges or edges in which material could be collected during the grinding procedure which would disable even mixing and grinding of the contents of the reaction vessel. The inner volume of the reaction vessel is approximately 14 cm ³ .

In mechanochemical experiment where the course of a mechanochemical reaction is in-situ monitored by means of Raman spectroscopy, reaction vessel 1 is, for performance of monitoring the flow of mechanochemical reaction on the mill, placed so that its flat transparent face 2 is horizontal and turned downwards. The laser probe is installed so that the incident laser beam enters the reaction vessel vertically through the flat face 2. The laser probe is also equipped with optical fibers that collect the scattered radiation and lead it to spectrometer for the purposes of analysis. The spectrometer is connected to the computer which, by means of specialized computer program records the spectra and enables their processing and storage. During the collection of a spectrum, reaction vessel is mounted on the crank of the mill and oscillates by typical frequency of 30 Hz in the horizontal plane so that the flat portion of the reaction vessel 2 is always horizontal and so that the laser beam is always vertical in relation to that flat face 2 in every position of the reaction vessel 1. The probe with laser is stationary during the collection of spectrum while above it the reaction vessel 1 oscillates.

By application of transparent reaction vessels 1 of the subject invention the recording of quality time- resolved Raman spectra during the grinding procedure is enabled and without halting the grinding procedure. Time resolution of spectra can be achieved ranging from millisecond to several seconds or minutes. The time resolution during the recording of time-resolved Raman spectra characteristic for certain mechanochemical experiment which need to be monitored in-situ, and is determined so that each certain spectrum that is collected is informative about the sample content, i.e. so that in the spectrum one can see contributions of different compounds which are to be found in the reaction mixture and so that changes in spectra which are collected at different time intervals can be observed. If the Raman scattering signal of the sample is weaker, it is necessary to increase time of collection of spectra whereas that time can be reduced if the sample or the reaction mixture gives strong Raman scattering of the incident laser radiation. Under the same conditions of recording, which are determined by the intensity of the incident laser beam and the time used for spectra collection, modification of the transparent reaction vessel which is the subject of this invention, what is achieved is collection of better quality Raman spectra which is manifested by the fact that the spectra collected by using the reaction vessel 1 of the subject invention have a better signal to noise ratio for the sample itself and a better signal ratio of the sample signal and the vessel signal than it is the case when using non-modified transparent reaction vessels.

When recording the efficiency of reaction vessel 1 of the subject invention, the same sample (benzoic acid) is recorded in three different ways: with laser entering vertically through the flat bottom of the reaction vessel 2, through the flat bottom of the reaction vessel 2, but not perpendicularly to the flat face 2, through curved (round) wall of reaction vessel 3. Other recording conditions (total time of measuring the spectrum, intensity of the incident laser beam) were identical in each of the three recordings.

Figure 3a shows that Raman signal of the sample, obtained by recording where the incident laser beam enters into the reaction vessel vertically through flat face 2 which is to be found at the outer mantel of the reaction vessel, is larger than the Raman signal of the sample which was obtained when the incident laser beam enters the reaction vessel through its curved wall 3. By the fact that the Raman signal of the sample is larger in case when spectrum is recorded so that the incident laser beam enters the reaction vessel 1 vertically on the flat face 2 at the outer mantel the ratio of Raman signal of the sample and noise and the ratio of the Raman signal of the sample and the reaction vessel is also better. From the above said, it follows that the recording of Raman spectra in a way that the incident laser beam enters the reaction vessel 1 vertically through the flat face 2 enables the collection of higher quality Raman spectra of the sample, which is desirable for the purposes of monitoring the course of mechanochemical reactions.

Raman spectra on drawings 4a, 4b and 4c show that besides the largest signal of the sample, which is the sum of intensities of signals FP3, FP4 and FP5, the recording perpendicularly through the flat face 2 has the best ratio of sample signal and reaction vessel signal. The noise of measuring shown on drawings 4a, 4b and 4c is approximately the same in all three measurements as data set out in Table 1 show. From that it arises that by recording of Raman spectra by using the reaction vessel 1 with flat outer wall, the best ratio of Raman signal and noise is obtained and thereby the best-quality Raman spectrum.

Table 1 - Overview of the difference of intensity and ratio of sample signal and reaction vessel signal with the incident laser beam entering perpendicularly through the flat portion of the reaction vessel, non- perpendicularly through the flat portion of reaction vessel and through the curved portion of reaction vessel.

Standard Standard Standard

Mean value deviation Mean value deviation Mean value deviation

Noise analysis of noise of noise of noise of noise of noise of noise in spectra intensity intensity intensity intensity intensity intensity

12.3 9.3 11.5 8.4 12.0 9.0

List of used positive marks

1 reaction vessel

2 flat transparent face

3 outer surface of reaction vessel

4 inner surface of reaction vessel

7 complementary parts of reaction vessel